VOL. LXII.J PHILOSOPHICAL TRANSACTIONa. ; 279 



adjusted by setting its edge exactly over the two marks made on each side the 

 short diameter of the aperture in the broad plates, and the cross thread must be 

 likewise set to agree with the strokes made on each side the longest diameter, 

 and then the intersection of the cross thread and the fixed parallel one, will be 

 the centre of the motion given to the outer plate of the micrometer (to which 

 the great screw index and threads are fastened) by the worm, by turning of 

 which the fixed parallel thread may easily be made to lie parallel to the apparent 

 motion of any object, in order to take the difference of declination and right 

 ascension from any other, that follows through the aperture of the telescope. 



This contrivance is of very great use to make a star, &c. move true along the 

 fixed parallel thread, which is absolutely necessary in order to take the true 

 difference of right ascension and declination between it and any other that follows. 

 Without this contrivance it is very difficult to make a star move exactly on the 

 thread, and it can only be done by repeated trials, which may sometimes take up 

 a great deal of time. 



If therefore a star be made to move on the parallel thread just at the cross, 

 and (the telescope continuing fixed in the same position) it be • afterwards, near 

 its going out of the aperture, found not to be on the thread, that must then be 

 brought to the star by the help of the worm, and then the thread will lie 

 parallel to the diurnal motion of the star in that part of the heavens, and conse- 

 quently the cross thread will represent a meridian, and the others parallels of 

 declination, and the difference of time between the passage of the star at the 

 cross wire (which was made to move along the thread), and the transit of any 

 other star, &c. over the cross thread which represents a meridian, turned into 

 degrees and minutes, will give the difference of right ascension. And if the 

 moveable parallel thread be brought, by turning the index, to touch the other 

 star about the time of its passage over the cross thread, then the number of re- 

 volutions and parts shown by the index, turned into minutes and seconds of a 

 degree by the table, will be the difference of declination between the two stars. 

 If the star be made to pass along the fixed thread so as to seem perfectly bisected, 

 there must be an allowance made for the semidiameter of the thread or wire, be- 

 cause he supposes the index to be adjusted as before to the inner edges of the 

 wires ; but it may, if found convenient, be adjusted to the middle of the threads, 

 or else correction may be made in the observed distance. 



In taking any angle, it is convenient that each of the parallel threads be about 

 the same distance from the middle of the aperture of the eye-glass , and for this 

 reason the whole micrometer is contrived to slide to and fro, as the case requires. 

 The same motion is also of use in taking the difference of right ascension and 

 declination, by sliding the fixed parallel thread (on which the preceding star is 

 brought to move) towards one side of the eye-glass ; for by that means a greater 



